To provide an insertion auxiliary fitting which can leave only a specified body of long lengths in an operable state on an occasion when a tubular passage is sealed off in a liquid-tight state by bringing a packing into close contact with the tubular passage from the outer peripheries of a plurality of bodies of the long lengths inserted through the passage.

The insertion auxiliary fitting 1 is used jointly on the occasion when a Y type connector 22 having the packing 24, or the like, is used. The fitting 1 has a first tubular part 2 which can be inserted into the tube bore of the Y type connector 2, in parallel with a guide wire 23, in a state wherein the catheter 20 is inserted therethrough, and which is provided inside with a valve disk 7 formed of an elastic member allowing the catheter 20 to be inserted slidably therethrough. The fitting 1 has further a second tubular part 3 which has a passage communicating with the first tubular part 2 and through which the catheter can be inserted. The second tubular part 3 make the catheter 20 free from the pressure of the valve disk 7 by expanding the valve disk 7.